I've been researching this for about a week and from what I understand it really depends on our Betta (marlin) if he even prefers tank mates or not. We've had him about 2 weeks and just upgraded him from a 1 gallon to a 10 gallon after reading more about what would be best for him. He seems to be a very happy, easy-going fishy.

Our plan is to get about 5 ghost shrimp to start and monitor how Marlin feels about them and whether or not they become lunch. However, I'd really like to have about 6 tetras in the tank as well if the ghost shrimp work out. I've read that neon tetras might not work out because they are too hyper and might nip Marlin's fins.

What types of tetras are best as tank mates or are there better fish that I'm not aware of?

Do you know your water parameters? pH and hardness will help determine what species would be better.
A general idea, ember tetra like soft water and low pH. X-ray tetra enjoy harder water and higher pH.
Do they have to be a tetra?

X ray tetra. There's also endlers live bearers, and you could have some platies.. Hardish water presents more challenges. But you will have longer lived fish and less problems if you chose a species that suits your water. :D
A bottom dweller like Cory pygmaeus, hastatus, or habrosus, would be a good choice, but these little guys need a soft sand bottom for their sensitive whiskers. Any of the smaller Cory species that suit your water are a good choice with sand.